- A FLASH Bootloader for PIC16 and PIC18 Devices--硬译（一）

两种器件系列的代码略有不同，但是功能本质是一样的。该目标实现的压力在于争取最大的性能和功能, 同时要求最少的代码空间。

FIRMWARE Basic Operation（固件基础操作）

图1  BootLoader 功能块图

Figure 1 summarizes （总结）the essential firmware design of the bootloader. 

图1总结了bootloader的基本固件设计。

Data is received through the USART module, configured in Asynchronous mode for compatibility（兼容性） 

with  RS-232 and passed through the transmit/receive engine. 

数据通过USART（串口）模块接收，配置为异步模式，通过发送/接收引擎下载，比如RS232。

The engine filters and parses the data, storing the information into a data buffer in RAM. 

引擎过滤并解析数据, 然后存储信息到在RAM的数据缓冲区中。

The command interpreter evaluates the command information within the buffer to determine what
should be done (i.e., Is the data written into a memory unit? Is data read from a memory unit? 

Does the firmware version need to be read?). 

命令解释器评估在缓冲区的命令信息来确定应该做什么。（比如：该数据是在存储单元以写进去的，

还是数据从该存储单元读取？固件版本是否需要读取？）

Once the operation is performed, data is passed back to the transmit/receive engine to be transmitted 

back to the source, closing the software flow control loop.

一旦操作方式被确定，被传递到发送/接收引擎的数据会被返送到宿主机，并关闭软件流控制回路。

COMMUNICATIONS （通信）
The microcontroller’s USART module is used to receive and transmit data; it is configured as a UART to
be compatible with RS-232 communications. 
微控制器的USART模块被用于接收和传送数据，它被确定用于UART去兼容与RS-232的通信。
The device can be set up in an application to bootload from a computer through its standard serial interface.
该器件能够通过它的标准串口从电脑引导加载建立起一个应用程序。
The following communications settings are used:
• 8 data bits
• No parity
• 1 STOP bit
下列通信设置将会被用到：
• 8个数据位
• 无校验位
• 1个停止位

The baud rate setting is variable depending on the application. Baud rate selection is discussed later.

波特率设置根据应用程序是可变的。波特率的选择稍后讨论。

THE RECEIVE/TRANSMIT BUFFER（ 接收和传送缓冲区）
All data is moved through a buffer (referred to as the Receive/Transmit Buffer). 　　
所有的数据都是通过一个缓冲区移动(称为接收/发送缓冲区)。
The buffer is a maximum of 255 bytes deep. 
缓冲区大小的最大值为255字节。
This is the maximum packet length supported by the protocol.
这个最大包长度有被协议支持。
 However, some devices may not support the largest packet size due to memory limitations. 
但是，一些器件由于内存大小并不支持该最大包长度。

Note: The actual packet length supported by a particular device depends on the size of its data memory.
注:  实际的包长度所支持的 特定器件取决于它的数据存储区的大小。

Figure 2 shows an example of the mapping of the buffer within the PIC18F452.
图2 显示了一个示例关于PIC18F452内的缓冲区映射。

图2 数据内存在PIC18F452的利用

A useful feature of the receive/transmit buffer is that it retains its memory between packets, thus 

allowing very fast repeat and replication operations. That is, if an empty packet is sent, the data